1.Field of the Invention
The present invention relates to an Mev ion implantation apparatus provided with an external resonance circuit type RFQ (radio frequency quadrupole) accelerator and, more particularly, to an RFQ accelerator enabled to decrease contamination of the substrate by heavy metallic particles when implanting ions into a substrate.
2. Description of the Prior Art
FIG. 2(A) shows a prior art example of an arrangement of an Mev ion implantation apparatus having an RFQ accelerator. An ion beam emitted from an ion source 1 is accelerated by an RFQ accelerator 2 up to an energy on the order of Mev, and thereby, ion implantation into a sample substrate 5 is performed. FIG. 2(B) is a drawing for describing a detailed arrangement of the RFQ accelerator 2. The RFQ accelerator has four electrodes 2a-2d shaped in an undulated form within its RFQ container 2e. Radio frequency power in the range from tens to hundreds of MHz is introduced into the cylindrical container 2e so that a radio frequency high voltage is induced at the undulated quadrupole electrodes 2a-2d, and the ion beam is accelerated by the thus generated radio frequency field.
The ion beam is accelerated when it moves along the center axis of the space surrounded by the quadrupole electrodes 2a-2d. As an example similar to the prior art example of FIG. 2(B), one disclosed in Japanese Laid-open Patent Publication No. 60-121656 may be mentioned.
In the prior art example, of various ion species emitted from the ion source, a specific ion species only is accelerated to be implanted into the sample substrate 5 and other ion species are generally diverged during their acceleration within the undulated quadrupole electrodes to collide with the electrodes. Hence, the electrode material (copper in general) is sputtered by the ion beam and particles of such heavy metal are deposited onto the sample substrate 5 to contaminate it. When the sample substrate 5 is made from a semiconductor material, in particular, the contamination by heavy metal has frequently caused fatal defects in the semiconductor characteristics leading to fabrication of inferior products.
Meanwhile, in the prior art example as shown in FIG. 2(B), it has been impossible to use other material than copper or analogous materials as the material for undulated quadrupole electrodes. This is because, when radio frequency power is introduced into the cylindrical container 2 made from other material than copper, its electric resistance becomes high and heat loss becomes great, thereby making it difficult to generate a radio frequency high voltage within the cylindrical container. In fact, it is known that the generated voltage is inversely proportional to the square root of the electric resistance r.
The prior art RFQ accelerator shown in FIG. 2(B) has a basic problem other than the problem of the contamination by heavy metal. That is, since the cylindrical container 2e in the prior art RFQ accelerator was formed so as to have a specific size constituting the so-called cavity resonator, the operating radio frequency was limited to a specific frequency, and therefore, when the ion species is determined, the accelerating energy is determined and, hence, there has been a problem that there is only one choice of accelerating energy. In order to have accelerating energy of variable values, it is required to have the exciting frequency made changeable at will. A prior art example whereby this purpose is attained is shown in FIG. 3 (refer to Japanese Laid-open Patent Publication No. 60-115199). Referring to the figure, undulated quadrupole electrodes 2a-2d are contained in the container 2e. This container, however, is a mere container and does not have a specific size to constitute a cavity resonator. A radio frequency high voltage supplied to the electrodes 2a-2d is generated in an electric resonance circuit formed of an inductor 9 and a variable capacitor 10. The electric resonance circuit is excited by an oscillator 6 through an amplifier 7 and a coupling coil 8. Variable frequency is obtained by changing the value of the capacitance C or inductance L, thereby changing the resonance frequency. That is, the electrodes 2a-2d are only supplied with the voltage. Therefore, the portion for which materials with low electric resistance should be used to facilitate generation of the radio frequency high voltage is limited only to the external electric resonance circuit formed of the inductance L, capacitance C, and so on. That is, different from the prior art example of FIG. 2(B), the radio frequency current in the resonance circuit does not flow into the undulated quadrupole electrodes. In FIG. 3, generation of the radio frequency high voltage can be sufficiently achieved by forming the variable capacitor 10, inductance coil 9, and wirings from such materials as copper or that coated with silver. Therefore, it is not required to fabricate the undulated quadrupole electrodes 2a-2d from a metal of particularly low electric resistance, but it may be made from materials not causing the contamination by heavy metal on the sample substrate 5.
In the case of the prior art example of FIG. 2(B), since it adopted the cavity resonator structure, it was essentially necessary to use low electric resistance material such as copper and silver for fabricating the undulated quadrupole electrodes 2a-2d, but such electrical restriction on the material of the quadrupole electrodes 2a-2d was first removed by the prior art example of FIG. 3. However, in connection with the prior art example of FIG. 3, there have not been made any considerations and devices, with a view to prevent the implanted sample substrate from being contaminated by heavy metal particles, as to selection of the electrode material and methods to prevent the contamination by heavy metal particles.